Since alloys capable of absorbing and desorbing hydrogen, or hydrogen storage alloys, were discovered, they have been applied to not only a hydrogen storage means but also a battery and the like. In particular, the hydrogen absorbing alloys developed with the intention of using them for a hydrogen car, an air conditioner and so on have undergone various improvements in recent years.
More specifically, a LaNi.sub.5 alloy which is first examined as a hydrogen absorbing alloy (See Japanese Tokkai Sho 51-13934, wherein the term "Tokkai" as used herein means an "unexamined published patent application") has the advantage of a high hydrogen storage capacity, but it suffers from disadvantages that not only La metal is expensive but also it is pulverized easily by repeating the absorption and desorption of hydrogen alternately and apt to be corroded by an alkaline or acidic solution.
Accordingly, when it is used as the electrode of an alkaline secondary battery, such an alloy can ensure a high initial electric capacity in the secondary battery. However, the electric capacity of the secondary battery is reduced to one-half or below by the charge-and-discharge cycles repeated about 50 times. Thus, such a secondary battery cannot stand a long use.
The aforementioned drawbacks of the LaNi.sub.5 alloy are mitigated by replacing a part of the La element by another rare earth element, such as Ce, Pr or Nd, and/or a part of the Ni element by another metal element such as Co, Al or Mn (See, e.g., Japanese Tokkai Sho 53-4918, Sho 54-64014, Sho 60-250558, Sho 61-91862 and Sho 61-233969).
The LaNi.sub.5 alloys modified as described above (hereinafter referred to as "LaNi.sub.5 type hydrogen absorbing alloys"), though they are somewhat inferior to the LaNi.sub.5 alloy in hydrogen storage capacity, have an advantage over the LaNi.sub.5 alloy in being improved in corrosion resistance to an alkaline or acidic solution. When they are each used for the negative electrode of an alkaline secondary battery, they can therefore lengthen the charge-and-discharge cycle life of the alkaline secondary battery. However, such a prolonged cycle life of the alkaline secondary battery, which is obtained by the use of the foregoing LaNi.sub.5 type hydrogen absorbing alloys, is still insufficient, and the electric capacity per unit weight is also unsatisfactory.
High capacity (or high electric capacity per unit weight) and long lifetime are the characteristics generally required for a battery. Thus, reduction in the reserve quantity of an electrode becomes necessary for the production of a secondary battery having a high capacity by the use of a LaNi.sub.5 type hydrogen absorbing alloy. Although the capacity of a battery can be elevated by the reduction of the reserve quantity, it is attended by a problem of shortening the cycle life. This problem is not yet solved.
When the secondary battery is overcharged, the oxygen gas generated from the positive electrode promotes the oxidation of a hydrogen absorbing alloy, and thereby the charge acceptance of the hydrogen absorbing alloy is lowered. As a consequence of the foregoing, hydrogen gas comes to be produced upon charging, and the hydrogen gas produced raises the internal pressure of the closed secondary battery to actuate a pressure valve, thereby causing a loss of the electrolytic solution. Thus, the internal resistance of the battery is increased; as a result, the discharge capacity is lowered as the charge-and-discharge cycle is repeated.
For the purpose of removing the foregoing drawback, the method of etching the hydrogen absorbing alloy with an acidic or alkaline solution and the method of plating the hydrogen absorbing alloy with copper or nickel have been proposed.
However, those methods are unsuccessful in the prevention of corrosion at the active surfaces attributable to cracks the hydrogen absorbing alloy newly has in it upon repetition of alternate charge and discharge, so that they cannot inhibit the alloy to lower its hydrogen storage capacity. Thus, it is difficult for those methods also to ensure a sufficiently long charge-and-discharge cycle life in the electrode.
With the intention of obviating the foregoing defect, the method of lengthening the charge-and-discharge cycle life of a battery has been proposed (Japanese Tokkai Hei 6-215765), wherein yttrium and/or a yttrium compound (inclusively referred to as "yttrium" hereinafter) is incorporated in a hydrogen absorbing alloy electrode; as a result, the yttrium is dissolved in an alkali electrolyte and deposited on active surfaces which are newly formed in the hydrogen absorbing alloy due to the generation of cracks, and the yttrium cover thus formed on the active surface inhibits the hydrogen absorbing alloy from undergoing oxidation to prevent the lowering of the hydrogen storage capacity. In addition, the method of incorporating a light rare-earth oxide in a hydrogen absorbing alloy in place of the foregoing yttrium compound has been proposed (Japanese Tokkai Hei 8-222210). However, the former method (Japanese Tokkai Hei 6-215765) is attended by deterioration in the initial activity, while the latter method (Japanese Tokkai Hei 8-222210) has a defect of being ineffective for hydrogen absorbing alloys other than those having laves phases with respect to the charge-and-discharge cycle life and the high temperature storage characteristics.